1. Field of the Invention
The present invention relates to a method of manufacturing a hydrophilic membrane having improved antifouling property and the hydrophilic membrane manufactured by the method.
2. Description of the Related Art
A process based on membranes is an energy saving type process which is not accompanied by phase, temperature and pressure changes that are inevitably accompanied in most separation processes, wherein importance of the process is greatly being magnified by combining the process of using membranes with various separation devices and developing materials for the membranes such that the process has recently been utilized in various fields including seawater desalination, food processing, various wastewater treatment, ultrapure water preparation, blood dialysis and filtration, and blood plasma separation. Solutes dissolved into various types of water such as colloid, bacteria, oil, protein, salts, viruses and others, or salts dissolved into water are capable of being removed using the membranes. The types of the membranes may include microfiltration membranes having a pore size of 0.1 to 100 μm, ultrafiltration membranes having a pore size of 0.005 to 0.5 μm, nanofiltration membranes having a pore size of 0.001 to 0.01 μm, reverse osmosis membranes having a pore size of greater than 0.001 μm, pervaporation membranes, gas separation membranes, etc., which are divided according to pore sizes and applications of the membranes. Membranes used particularly in the water treatment process or liquid/solid separating membrane process include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes. Important factors characterizing liquid/solid separating membranes showing superior performance may include excellent permeation flux, high selectivity and antifouling property.
Membranes having high water permeation flux are economically efficient by reducing the energy cost of pumping because water can penetrate through the membranes at a relatively low pumping pressure. In addition, membranes having uniform micropores can have high separation efficiency because separation efficiency with respect to the solute size are higher in the membranes having uniform micropores than in membranes having non-uniformed micropores.
Fouling of the membranes is one of the most important factors determining economic efficiency of the membrane-based separation process. Membrane fouling is characterized in that concentrates or excluded components such as proteins, cells, colloids and others dispersed or dissolved in a solution are adsorbed onto surface and micropores of the membranes, which results in a sharp decline of the permeation flux in the course of permeation period. The membrane fouling is caused by external fouling in which the excluded components are adsorbed onto the membrane surfaces to form a gel layer, a cake layer, a scale layer and other layers, and internal fouling causing adsorption of membrane pores and closure of the membrane pores. If such surface and micropores of the membranes are fouled, there is a problem that the permeation flux is rapidly decreased or properties of the membranes are varied with the passage of time to result in a deteriorating separation function of the membranes. A method of eliminating fouled contaminants adhered to the membranes at predetermined time intervals using a backwashing process and an air cleaning process individually or in combination in order to extend the life of the membranes has been used. However, in conclusion, the method results in deteriorating economic efficiency of the membrane process since much of energy is consumed due to frequent cleaning, a constant permeation flux cannot be obtained, and the membranes should be replaced due to impossible performance recovery of the membranes.
Polymeric materials for the membranes are mainly divided into hydrophilic polymers and hydrophobic polymers. The hydrophilic polymers for membrane materials may include cellulose-based polymers such as cellulose acetate, cellulose nitrate and the like, and polyamide-based polymers such as nylon and the like. The cellulose-based membranes have drawbacks that the cellulose-based polymers are very sensitive to heat, chemical resistance of the cellulose-based polymers is low, and main chains of the cellulose-based polymers are easily cleaved by enzyme and the like, although the cellulose-based polymers have characteristics that water is easily penetrated through the cellulose-based membranes by the interaction between water and the cellulose-based membranes such as hydrogen bonds and the like. On the other hand, the polyamide-based polymers are widely used particularly as materials for the reverse osmosis membranes due to their excellent mechanical properties, thermal stability and hydraulic stability. However, the polyamide-based polymers have drawbacks that it is difficult to manufacture polyamide-based polymers for microfiltration membranes or ultrafiltration membranes, and the polyamide-based polymers are strongly bonded with proteins to result in a severe fouling of the membranes.
The hydrophobic polymer materials may include polyethylene (PE), polypropylene (PP), polycarbonate (PC), polyimide (PI), polyetherimide (PEI), polysulfone (PSF), polyethersulfone (PES), polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), and the like. The hydrophobic membranes are widely being used as materials for water treatment membranes due to their inherent advantages of excellent durability, mechanical strength, thermal stability and chemical resistance. However, the hydrophobic membranes have a drawback that a high trans-membrane pressure should be applied to allow water penetration through the hydrophobic membranes because there are no chemical functional groups that are capable of obtaining good interaction between water and the membrane such as hydrogen bonds and the like such that the hydrophobic membranes are not easily wetted by water. Furthermore, the hydrophobic membranes have a drawback that the hydrophobic membranes are extremely sensitive to membrane fouling compared to the hydrophilic polymer membranes.
In order to overcome such drawbacks, a method of hydrophilicizing surfaces of the hydrophobic membranes to retain beneficial bulk properties of the hydrophobic membranes such as chemical resistance, mechanical strength, thermal stability and chemical stability, and beneficial surface properties of the hydrophobic membranes such as high permeation flux and antifouling property has been suggested. The method typically includes a physical coating method, a polymer blending method, a grafting method, a coating method using a cross-linking reaction, a chemical treatment method, and others.
A physical coating method is a method of physically modifying the surface of the membranes by coating surface of hydrophobic membranes with hydrophilic polymer materials such as poly(vinyl pyrrolidone), poly(ethylene glycol), poly(vinyl alcohol), and the like. The physical coating method has a drawback that it is difficult to coat the internal micropores of the membranes uniformly because of the high surface tension and the high viscosity of coating solvents, typically water. In addition, high expenses are often required for post-treatment of the used coating solvents. Moreover, the physical coating method has a drawback that the hydrophilic polymer material is separated from the hydrophobic membrane surface during the membrane operation, resulting in lost of hydrophilicity and a drawback that the separated material is mixed with a filtrate, resulting in a contamination of the filtrate accordingly since the hydrophilic polymer material is dissolved in water when the manufactured membranes are brought into contact with water.
As disclosed in WO98/08595, and U.S. Pat. Nos. 5,066,401, 4,302,334, 5,122,273 and 5,503,746, polymer blending methods using hydrophilic polymers such as poly(ethylene glycol) and poly(vinyl pyrrolidone) have drawbacks that hydrophilicizing effects are insignificant when blending a small amount of hydrophilic polymer to form membranes, and film-forming conditions are varied to make it difficult to control pores of the membranes when using a large amount of hydrophilic polymer since hydrophilic polymer is present on bulks as well as surfaces of the membranes after forming the membranes. Further, the polymer blending methods have drawbacks that phase separation phenomena are generated due to inherent immiscibility between hydrophilic polymers and hydrophobic polymers, and the hydrophilic polymers are eluted during a long-term operation to result in a variation of filtration properties.
The grafting method is a method of hydrophilicizing the membranes by contacting the membranes with a solution containing acrylate monomers having hydrophilic groups and inducing a grafting reaction onto the surface of the membranes after forming radicals on the surface of membranes by irradiating high energy sources such as ultraviolet (UV) rays, electron beams (EB), ozone, plasma, gamma-rays and others as disclosed in U.S. Pat. Nos. 4,311,573, 5,019,260, 5,736,051, 6,280,853, and 7,607,058. However, the grafting method is not economically efficient since relatively expensive irradiation methods using high energy sources such as electron beams, plasma, gamma-rays and others should be used in the grafting method, and structure of an apparatus for the grafting method is complicated, and the grafting method has a problem that the membranes are destroyed permanently during irradiation of the high energy sources to result in decreased mechanical strength of the membranes. Further, the grafting method has a drawback that it is difficult to decrease internal fouling of the membranes since high viscosity and surface tension of the solvents make it difficult to penetrate the solvent into pores of the membranes and to perform uniform grafting into the inside pores of the membranes when using hydrophilic solvent such as water, alcohol or the like as the grafting solvents. The grafting method further has a drawback that micropores of the membranes are often clogged when grafting is excessively attempted to overcome the internal fouling of the membranes.
A coating method using a cross-linking reaction is a method of introducing hydrophilicity into the membranes by contacting the dissolved solution to the surface of the membranes and coating the surface of the membranes with the dissolved solution by a cross-linking reaction using heat, ultraviolet rays, electron beams or the like after dissolving a hydrophilic monomer, a cross-linking agent, an initiator and others into water or alcohol as disclosed in U.S. Pat. Nos. 4,994,879, 6,618,533 and others. The foregoing coating method has an advantage that the hydrophilic material is not eluted when hydrophilic material is brought into contact with water. However, the coating method has a disadvantage that it is difficult to uniformly coat micropores of the membranes because wettability of hydrophobic membranes with water or alcohol is very low. The coating method has disadvantages that the entire process becomes complicated, and the manufacturing cost is increased when a compression process using rollers is applied to overcome non-uniform coating in the micropores of the membranes.
Therefore, the present inventors have made an effort to solve the above-mentioned problems of the prior art. As a result of the effort, the present inventors have completed this invention by developing a method which is capable of uniformly coating surfaces and pores of the membranes using a supercritical fluid or a subcritical fluid that has excellent wettability and is easily penetrated into micropores of the membranes as a coating solvent, and which is capable of hydrophilicizing the entire membranes permanently through a cross-linking reaction.